Don't forget to check if the cards have a dual bios switch.
My RX580s didn't have any and quite a few of them got bricked and they are of no use now.
It'll give you a failsafe when you'll be bios modding and also backup the bios for all the cards one by one.
Don't make the same mistakes I made.

You won't have to worry about cooling them or heat if you're properly undervolting them.
At most my cards were hashing at 31 MH/s with 75W consumption with 60 C in an ambient temp of 24 C.